This invention relates to a bias circuit, and more particularly to a bias circuit suitable for a power source for supplying a bias voltage to be applied to a semiconductor integrated circuit.
If, for example, an output level of a semiconductor integrated circuit is varied according to a power supply voltage variation or ambient temperature change, it becomes difficult for the integrated circuit to maintain a required noise immunity and accordingly to assure stable operation of the circuit. Therefore, a bias circuit for supplying a bias voltage to a semiconductor integrated circuit should be constructed so that even if the input voltage applied to the bias circuit is varied, the bias voltage, will not vary. However, the bias voltage may be varied with the fluctuation in ambient temperature so as to compensate for a temperature characteristic in the output level of the semiconductor integrated circuit. Various proposals have been advanced to solve the problem (e.g. "Fully Compensated Emitter Coupled Logic", H. H. Muller, et al., ISSCC Digest of Technical Papers, p. 168-169, FIG. 2, February 1973, or "A Simple Three-Terminal IC Boundgap Reference", A. P. Brokaw, IEEE Journal of Solid State Circuits, vol. sc-9, no-6 p. 388-393, December 1974). A known bias circuit which has been proposed for satisfying the aforesaid requirements comprises a voltage regulator and a temperature compensating circuit. An output voltage from the voltage regulator is applied to the temperature compensating circuit, as a power supply voltage, to impart a required temperature characteristic to the voltage regulator. The temperature compensating circuit imparts a temperature characteristic that will compensate for a temperature characteristic of the semiconductor integrated circuit. This bias circuit, however, has a disadvantage of possibly causing oscillation due to a positive feedback loop formed therein.
Therefore, it is difficult for such a conventional bias circuit to expect a stable operation. In view of this, the assignee of the present application has previously proposed a bias circuit which does not include a positive feedback loop, but can achieve temperature compensation (Japanese Utility Model Application No. 52-177560). However, in this bias circuit, the circuit for imparting the required temperature characteristic is directly affected by power supply voltage variation, so that it is impossible to satisfactorily carry out a voltage regulating operation.